1. Field of the Invention
This invention relates to a biodegradable plastic composition superior in both biodegradability and physical properties, a method for preparing thereof and product prepared therefrom. More particularly, the present invention relates to a biodegradable plastic composition in which thermoplastic modified starch is chemically bonded to a matrix resin comprising polyethylene having a good processability and physical properties and biodegradable aliphatic polyesters. Also, the present invention pertains to a method for preparing the biodegradable plastic composition using a twin screw extruder, and pertains to a product prepared from the biodegradable plastic composition.
2. Description of the Prior Art
Synthetic plastics have been considered to overcome the limits and restraints of natural materials by virtue of their excellent physical properties and low price. It may fairly be said that various synthetic polymers including plastics have greatly contributed to the modern scientific civilization in many fields.
Plastic products, now pouring out from all over the world, however, cause such serious pollution as threatens the future of mankind. To solve this problem, various countermeasures have been taken. Conventionally, the methods of disposal of solid wastes including plastics include burial, incineration and recycling. However, these means cannot be a complete solution to the environmental pollution problem at all.
Recently, active research and development efforts have been directed to so-called degradable plastics which are environmentally self-degradable. There are three groups of degradable plastics: biodegradable plastics, photo-degradable plastics and bio- and photodegradable plastics of these, biodegradable plastics have been most widely used. This invention is also subject to utilization of biodegradable plastics.
Biodegradable plastics developed so far comprise as a degradable material, polymeric products of microorganisms such as poly-.beta.-hydroxybutylate, polymers synthesized from the biochemicals produced by microorganisms, chemically synthesized aliphatic polyesters, or naturally synthesized polymers such as chitin, starch, etc.
The prior art concerning biodegradable plastics is exemplified by EP No. 304,401 in the name of Warner-Lambert Co. This patent discloses a processing technique that when starch containing a certain amount of moisture is kneaded with a plasticizer at a high temperature of 150.degree. C. or higher in a closed system such as an extruder, the vapor pressure increases to break the hydrogen bond between starch molecules thus producing starch having thermoplasticity. This technique is mainly devoted to the production of molded parts. However, it is not suitable for film production. A high melting strength is required in processing film, and high elongation and tensile strength are necessary after production.
EP Nos. 400,531 and 400,532 to Novamont, introduced a technique by which, when a synthetic resin compatible with starch, such as ethylene-vinyl alcohol copolymer and ethylene-acrylic acid copolymer, is mixed with starch containing a certain amount of water in the presence of a plasticizer at a high temperature of 150.degree.C. or higher in an extruder, intermolecular bonds of starch are destructurized and the starch chemically and physically binds to the resin to produce biodegradable films containing a high starch content. But the synthetic resins such as ethylene-vinyl alcohol copolymer and ethylene-ethylacrylic acid copolymer, display no biodegradability and are very expensive, so that it is quite difficult to substitute the films prepared from these synthetic resins for plastic films for general uses.
In addition, since aliphatic polyesters are expensive, they can be blended with low-priced polyethylene to reduce material cost. But the two polymers are not compatible with each other and thus the-mechanical properties decrease greatly by simple mixing. In the case of aliphatic polyesters having a low melting point of about 60.degree. C., such as polycaprolactone, it is impossible for them to be processed into blown films, because the crystallization temperature is so low that bubbles of the films can not be sufficiently cooled under atmospheric air condition. So an additional cooling system like Chiller is necessary.
Korean Pat. Laid-Open Publication Nos. 94-11556 and 94-11558, both to the present inventors, disclose that starch is chemically bonded to the matrix resin by a coupling agent to overcome the deterioration of mechanical properties due to the addition of starch and to reduce the cost price by simplifying the procedure. However, there are still some problems such as low biodegradability and deterioration of mechanical properties due to the addition of starch.